1. Field of the Invention
The invention relates to a semiconductor device and a method of fabricating the same, and more particularly to an improvement in an interlayer insulating film suitable for a semiconductor device having multi-layered wirings.
2. Description of the Related Art
Recently, integration in a semiconductor device has been significantly enhanced. With such enhancement in integration, a semiconductor device is designed to have a multi-layered wiring structure including a growing number of wiring layers. A semiconductor device is designed to have an insulating film around or between wiring layers for prevention of current leakage. In particular, an insulating film formed between wiring layers is called an interlayer insulating film. As mentioned above, under the circumstance that steps on a surface of a semiconductor substrate on which a wiring layer(s) is(are) formed are increased with a wiring layer in a semiconductor device being designed to have a multi-layered wiring structure having the growing number of layers, it is indispensable that such surface steps are planarized by means of an interlayer insulating film.
In order to planarize steps formed on a surface of a semiconductor substrate, the following method is presently and widely used in the field of semiconductor fabrication.
First, a wiring pattern made of aluminum is formed on a semiconductor substrate such as a silicon substrate, and then, a film made of ozone (O.sub.3) tetra etyl ortho silicate (TEOS) boron phospho silicate glass (BPSG) is formed all over the semiconductor substrate by normal pressure chemical vapor deposition (CVD). Then, the semiconductor substrate is annealed, for instance, at 800.degree. C. to 900.degree. C. in nitrogen (N.sub.2) atmosphere to thereby reflow the O.sub.3 TEOS BPSG film.
One conventional method of forming a O.sub.3 TEOS BPSG film is illustrated in FIGS. 1A to 1C.
First, as illustrated in FIG. 1A, a polysilicon film 11 is formed on a silicon substrate 10 and patterned. Then, a film 12 made of O.sub.3 TEOS BPSG containing boron (B) and phosphorus (P) at a relatively low concentration is formed over the patterned polysilicon film 11 and the silicon substrate 10 by normal pressure CVD.
Then, as illustrated in FIG. 1B, a film 13 made of O.sub.3 TEOS BPSG containing boron (B) and phosphorus (P) at a relatively high concentration is formed over the O.sub.3 TEOS BPSG film 12.
Then, as illustrated in FIG. 1C, the silicon substrate 10 is subject to heat treatment by exposing to nitrogen (N.sub.2) atmosphere at 800.degree. C. to 900.degree. C., to thereby reflow the O.sub.3 TEOS BPSG films 12 and 13.
Japanese Unexamined Patent Publication No. 4-94539 has suggested another method of planarizing surface steps of a semiconductor substrate. In the suggested method, a surface of a substrate is first reformed by irradiating high frequency plasma thereto. Then, there are formed PSG film, BSG film and BPSG film on the substrate under normal or lowered pressure by employing a mixture gas containing an organic silane (SiH.sub.4), a gas containing impurities such as phosphorus and boron, and ozone.
Japanese Unexamined Patent Publication No. 5-259297 has suggested still another method of planarizing surface steps of a semiconductor substrate. In this method, prior to formation of an interlayer insulating film, there is formed a nitrogen containing layer which is to be disposed just below the interlayer insulating film.
However, the above-mentioned prior methods have a problem as follows.
It is well known that a O.sub.3 TEOS BPSG film, formed by normal pressure CVD, containing boron (B) and phosphorus (P) at higher concentration would provide more excellent planarization of surface steps on a substrate. On the other hand, if a O.sub.3 TEOS BPSG film contains boron (B) and phosphorus (P) at high concentration, there takes place a phenomenon that boron and phosphorus particles are precipitated at a surface of the O.sub.3 TEOS BPSG film. Such precipitation of boron and phosphorus particles on a surface of a O.sub.3 TEOS BPSG film exerts a harmful influence on subsequent steps, resulting in reduction in a yield. That is, the above-mentioned prior methods have a problem of a narrow margin with respect to boron and phosphorus concentrations.
In other words, the prior methods have a problem that it is not allowed to let a O.sub.3 TEOS BPSG film contain boron and phosphorus at high concentration because of low limit thereof about precipitation of boron and phosphorus, resulting in that the O.sub.3 TEOS BPSG film cannot be sufficiently planarized after being reflowed.
To the contrary, if a O.sub.3 TEOS BPSG film were designed to contain boron and phosphorus at higher concentration in order to sufficiently planarize the O.sub.3 TEOS BPSG film by reflow process, it would be accompanied with a problem that boron and phosphorus particles precipitate at a surface of the O.sub.3 TEOS BPSG film.